The present invention relates to layer insulation for use in high-voltage electrical equipment and more particularly to such layer insulation comprising substantially all inorganic material.
Layer insulation has a generally planar configuration and is used in high-voltage electrical equipment such as distribution transformers to provide insulation between adjacent "layers" of a winding thereof. (As used herein, the term "high-voltage" is intended to signify a typical power level voltage.) Layer insulation is typically impregnated with oil to improve the dielectric and heat removal properties thereof. A known layer insulation comprises cellulose paper. Cellulose paper is subject to various disadvantages when used as layer insulation. First, cellulose paper has low hydrolytic stability, or resistance to degradation by water which may come into contact with cellulose paper. Second, cellulose paper degrades rapidly at temperatures in excess of about 165.degree. C. which can occur in high-voltage electrical equipment such as a distribution transformer due to electrical overloads or to localized "hot spots" in a winding thereof. At such temperatures, cellulose paper becomes brittle and, further, the resistance of the paper to degradation by water becomes lowered. Additionally, prolonged exposure of cellulose paper which is impregnated with oil to temperatures in excess of about 180.degree. C. causes the paper to become carbonized due to the lack of oxygen. This can occur with cellulose paper because it is organic material and therefore contains the carbon atoms necessary for carbonization. Because carbonization results in a path of relatively high conductance, a severe degradation of the dielectric property of the cellulose paper occurs.